Molecular Dynamics Simulation of Micro Friction Behavior of Graphene Loading Copper Nanoparticles

Nanoparticles significantly provides many powerful functions and a friendly use to improve the tribological performance of sliding components lubricating oil additives. Nevertheless, most analysis for the lubrication mechanism of nanoparticles is based on experimental phenomena, which is fuzzy to verify the lubrication mechanism with direct evidence, and further, it is difficult to design the lubrication efficiency according to the requirements.

In order to investigate the synergistic mechanism of friction reduction and anti-wear mechanism for graphene loading copper nanoparticles (RGO/Cu), the motion state of RGO/Cu between friction pairs is studied from the micro point of view, the lubrication mechanism of RGO/Cu as lubricant additives is analyzed. In this study, the molecular dynamics model of Cu-Cu tribo-pairs with three-layer and single-layer RGO/Cu and liquid argon as the basic fluid was established based on the molecular dynamics theory. The Lennard-Jones (LJ) potential function was used to describe the interactions between AR-AR, AR-C, AR-Cu and C-Cu atoms, the EAM potential function was used to describe the Cu-Cu interaction, and the Teroff potential function was used to describe the C-C interaction force. The Velocity Verlet algorithm was used to simulate the micro movement and the friction process of RGO/Cu in basic fluid between Cu-Cu tribo-pairs under different loads. The results showed that the movement of copper particles between the tribo-pairs had both translation and rotation, and the copper particles had fission migration, while graphene was only translation, which was easy to adsorb on the surface of the tribo-pairs to form a protective film. Compared with three layers RGO/Cu, single layer RGO/Cu had larger migration and deformation in the basic fluid, and the rotation and migration of copper particles between tribo-pairs were also more intense. With the increase of load, the fission and migration of copper particles became more and more obvious, and some copper atoms falled off and interacted with the tribo-pairs and adhered to the surface of the tribo-pairs.

Finally, using paraffin as the base oil, the friction-reduction and anti-wear performance of RGO/Cu additive were verified by experiments. The maximum depth of the RGO/Cu oil lubrication wear scar was 43.6% and 16.7% less than that of the base oil and graphene oil, respectively. The widths of the wear scars for the RGO/Cu oil were 62.3% and 55.3% less than that of the base oil and graphene oil, respectively. The mean roughness on the wear scars of the RGO/Cu lubrication were 57.0% and 26.5% less than that of the base oil and graphene oil, respectively. The element C on the friction surface lubricated with RGO/Cu oil increased from 6.00% to 9.07%, and the characteristic element Cu of it increased from 0.11% to 0.37%. The friction coefficient for the RGO/Cu oil tended to decrease with the increase of load. The molecular dynamics theoretical simulation results were consistent with the model experimental results. The theoretical and experimental results showed that high pressure condition was easier to exert synergistic friction-reduction and anti-wear performance for the RGO/Cu additive.